by E.H. Shepard from A. A. Milne's Winnie-the-Pooh books

Local Supernova Rate

Barbara Jo Mattson

In 1974, Ruderman showed that if a supernova (SN) occurred sufficiently close to the solar system (nearer than about 10 parsecs), it could disrupt life on Earth.  Two phenomena resulting from such a nearby SN would severely deplete the Earth's ozone layer allowing more harmful radiation to reach the Earth's surface.  First, there would be an initial burst of gamma-rays from the SN (not a gamma-ray burst unless the supernova happened to be of a very special type, which is highly unlikely) lasting about three months.  Second, since cosmic rays are thought to be accelerated in the shock wave of a SN (the supernova remnant), there would be an increase in cosmic ray activity on Earth lasting from 1,000 to 10,000 years.  Such an event, could explain a few of the mass extinctions which have occurred through Earth's history.  For this idea to remain a viable explanation for mass extinctions, we must know how often a SN occurs within 10 pc of Earth.

Surprisingly, over the past 25 years, not much has changed in attempts to determine the local SN rate.  The main difference between estimates of 25 years ago and today is the Milky Way's Galactic SN rate.  Other than that, two methods for determining the local SN rate are commonly employed.

SN Distributed Throughout Milky Way's Disk

One method is to assume that SN in the Milky Way occur anywhere in the galactic disk.  This is reasonable because the disk is where the young, hot, massive stars likely to "go supernova" reside.  The easiest treatment is to assume the disk is a cylinder so that SNe are as likely to happen in one part as the next.   A slightly more difficult (though still fairly simple) treatment is to take into account the exponential distribution of stars above (and below) the central plane of the galaxy.  In this second case, SNe are a bit more probable near the center of the galactic plane than too far above or below.

Using one model or the other, we can calculate a volume in which SN will occur.  To find the rate of SN within a certain radius, r, we just divide the rate by the volume of the Milky Way and multiplies by the spherical volume enclosed by r.  The only unknown here is the Galactic SN rate, which will be discussed later.

SN Confined to Milky Way's Spiral Arms

Two facts justify confining SN to be within the Galaxy's spiral arms in the local SN rate calculation.  First, star birth in the Milky Way (and other spiral galaxies) is confined to the spiral arms.  Second, the massive stars responsible for SNe have short lives, and thus will not travel far from their birth place.

The sun passes through the spiral arms about every 108 years (100 Myr) and takes about 107 years to pass through the arm.  Using the Galactic SN rate and the volume occupied by the spiral arms, we could then estimate how often a SN would occur within a certain radius of the solar system as it passes through the spiral arm.  Once again, we need the Galactic SN rate.

Milky Way's Galactic SN Rate

Estimates for the Milky Way's Galactic SN rate are based on large studies of other galaxies.  It is thought that galaxies of a similar Hubble type will have similar SN rates; therefore, if we could determine the SN rates of other galaxies like our own, we could determine the Milky Way's rate.

In order to observe a statistically significant number of galaxies of the various Hubble types, studies of galactic SN rates must probe the deepest regions of the Universe.  Unfortunately, this adds the complication of Hubble's constant (H0).  Currently values of H0 range from 65 - 75 km s-1 Mpc-1 (some values are still quoted as low as 50 and as high as 100, but most values lie in that given range).  We will adopt a Hubble type for the Milky Way of Sbc and a Galactic luminosity of 2.3 x 1010 Lsun,b.  Then, according to a study published by Cappellaro, Evans and Turatto (1999), the Milky Way's Galactic SN rate will be one every 67 years for H0 = 65 km s-1 Mpc-1,  one every 57 years for H0 = 70 km s-1 Mpc-1 or one every 50 years for H0 = 75 km s-1 Mpc-1.

A second method for determining the Galactic SN rate is to look at galactic SN remnants, historical SNe and novae in M31 and M33 (our closest neighbor galaxy and its companion dwarf spiral galaxy).  Using such a method, van den Bergh and McClure (1990) find a galactic SN rate of two per century (50 years between SN) - quite in line with the above quoted values.

SN Rate Within 10 pc

Naturally, the two methods for estimating a local SN rate yield widely different answers.  Using a SN rate of one per century and confining SN to the spiral arms, Clark, McCrea and Stephenson (1977) find that a SN would occur within 10 pc each time the Sun passes through a spiral arm - so every 108 years.  The SN rate is consistent with H0 = 75 km s-1 Mpc-1.   Using a SN rate to 1.5 per century (H0 = 65 km s-1 Mpc-1) decreases the time between nearby SN by about 0.7 (so every 70 million years).

Whitten et. al (1976) estimate the SN rate by ignoring the concentration of SN in the spiral arms, but do take account the observation (from Clark and Caswell (1976)) that local SN have a volume density one quarter that of SN in the inner Galaxy.  Using a Galactic SN rate of two per century, they find that SN will occur within 10 pc of Earth every 109- 1010 years (again, consistent with H0 = 75 km s-1 Mpc-1).  Using a Galactic SN rate of 1.5 per century (H0 = 65 km s-1 Mpc-1) will increase the time between nearby SN by about 1.3 (so 1.3 Gyr - 13 Gyr).

Results from these studies have been quoted even just a couple years ago by Gonzalez (1999), with debate only over the Galactic SN rate.  So these methods have held up for over 25 years - our knowledge of galactic astronomy being the only fluid factor.

We can reasonably say that such events are not likely to occur more often that every 100 Myrs (and perhaps much less often).  However, this does show that a nearby SN within the history of Earth is likely to have occurred at least once and up to 40 times.  These could, therefore, account for a few, though not most, of Earth's mass extinctions.

References

Created: 05 July 2001